A good quality napped fabric is one where yarns in the fabric are evenly napped. To assist in napping, it is desirable to provide face yarns which are easily napped. Easily napped fibers allow greater control of the degree of napping and are cost efficient. Therefore, yarns destined for napping should have physical properties suited to napping. Where yarns require more than one pass through the napping machine to prepare a good quality napped fabric, the cost of producing the napped fabric increases for each pass through the napping machine.
One way of making a yarn which is easily napped is by decreasing the tensile strength of the yarn fibers. Known methods for decreasing yarn strengths include the addition of additives to a polymer, for example, to form a copolymer which reduces the molecular weight of the polymer to correspondingly reduce intrinsic viscosity with concomitant decrease in the tensile strength of the thread. The use of such relatively low intrinsic viscosity (IV) polyesters is known to be advantageous for certain applications. For example, U.S. Pat. No. 3,808,302 to Dyer et al. discloses a method of reducing pilling in polyester fabrics by reducing the intrinsic viscosity of the fabric. The resultant fiber has an ultimate tenacity of about 2.6 to 3.2 grams per denier and elongation of about 25 to 40 percent. The lowered strength due to the lowered intrinsic viscosity helps to prevent the formation of pills.
In addition, it is known to chemically weaken polyester fibers, as well as fabrics manufactured from them, by treating the fiber or fabric with a weakening agent. For example, U.S. Pat. No. 4,004,878 to Magosch et al. describes treating polyester threads with aqueous acid to reduce their strength and, in turn, reduce pilling.
It is known also to tailor polyester products to certain end uses, including napped fabrics. Japanese Kokai Patent Application No. 58-104217 describes a multifilament poly(tetramethylene terephthalate) of 0.3 to 0.9 denier and various other properties for ultrafine applications. This fiber is useful in making tricot and tricot raised commodities (napping). The Kokai describes ultrafine polyethylene terephthalate fibers as not having all the desired physical properties required for tricot.
Japanese Kokai Patent Application No. 61-194211 describes a poly(ethylene terephthalate) product which is drawn at 160.degree. C. and 5000 mpm to give fibers having 60% elongation, tensile strength of 1.4 g/denier and boiling water shrinkage of 3.5%. The yarn is used at relatively high speeds in a water jet loom without gripper breakage or reed wear.
Japanese Kokai Patent Application No. 61-194212 describes a polyethylene terephthalate yarn having an elongation of up to 80%, tensile strength of about 1.4 g/denier and boiling water shrinkage less than 5%. The yarn is prepared at high speeds (4000-8000 mpm) for use in linings, etc.
U.S. Pat. No. 4,970,038 to Stanko describes a process for making moderate tenacity polyester (about 4.5-7.0) with boiling water shrinkage of about 2% to 10%.
Relatively high speed processes for preparing polyethylene terephthalate yarns are known. In addition to high speed processes mentioned above, U.S. Pat. No. 4,003,974 to Chantry et al. describes a high speed process for preparing a high tenacity (7.5 to 9 grams per denier), low elongation (12 to 20 percent) fiber. These yarns have a dry heat shrinkage of 4 percent or less. The patent describes wind-up speeds of 2,000 yards per minute and higher.
Yet, there remains a need for polyethylene terephthalate yarns which are suitable for fabrication into fabrics and are still easily napped.